The field of the invention relates to force transducers wherein an applied force, such as an acceleration force or a fluid pressure force, is converted into an electrical signal. In particular, the field of the invention relates to force transducers and methods for producing such transducers by etching silicon substrates.
Force transducers are known having a suspended mass, such as a pendulum or cantilever, which deflects in response to an applied force. typically, capacitive plates are coupled to surfaces of both the suspended mass and an adjacent structure. As the suspended mass deflects, the resulting change in capacitance provides an electrical indication of the applied force.
An example of an accelerometer is disclosed in U.S. Pat. No. 4,679,434 issued to Stewart et al. A cantilever is formed by etching a substantially U-shaped cavity through a silicon wafer. Two Pyrex.TM. planar surfaces are then attached to opposing planar surfaces of the substrate such that the cantilever is suspended therebetween. Conductive plates are bonded to both Pyrex.TM. surfaces and the suspended mass. A differential amplifier applies a voltage to the plates in response to detected deflection of the suspended mass for restoring the suspended mass to its null position. The applied voltage is, allegedly, proportional to the applied force.
The inventor herein has recognized numerous disadvantages of the prior approaches. In motor vehicle applications, for example, accelerometers are deployed in air bag systems wherein false triggering due to noise is intolerable. When the motor vehicle traverses rough road surfaces, there may be deflection of the suspended mass due to noise resulting in erroneous interpretation as a collision. Since the prior approaches appear to have the ability of sensing displacement only in the time frame of motion of the suspended mass, their ability to discriminate against vehicular noise is limited. An additional disadvantage is that only a portion of the structure disclosed may be fabricated by silicon processing technology. Thus, the potential advantages of batch processing technology are not fully utilized. An additional disadvantage is that deflection of the suspended mass is essentially arcuate. Accordingly, capacitive changes and corresponding electrical measurements are nonlinear with respect to the applied force.